Characteristics and Genesis of Collophane in Organic-Rich Shale of Chang 7 Member in Ordos Basin, North China

(1) Background: The Ordos Basin is one of the sedimentary basins in China that is richest in oil and gas resources. The Chang 7 member of the Yanchang Formation is a set of organic-rich shale, abundant in collophane. (2) Methods: The observation and analysis of rock thin sections, combined with major elements, trace elements, electron probes, and other technical means, the characteristics and genesis mechanism of collophane in the organic-rich shale of the Chang 7 member of the Yanchang Formation in the Ordos Basin were studied. (3) Results: Collophane are divided into oolitic collophane, red-yellow aggregate collophane, and apatite-containing crystalline collophane; the main chemical compositions of the collophane were CaO, P2O5, FeO, Al2O3, and MgO. (4) Conclusions: Phosphorus elements of collophane in the organic-rich shale of the Chang 7 member of the Ordos continental lake basin are mainly derived from the nutrients carried by the volcanic ash sediments around the basin and the hydrothermal fluid at the bottom of the lake. The formation of collophane is divided into two periods: during the sedimentary period, the phosphorus released by the aerobic decomposition of phytoplankton to the mineralization and degradation of organic matter, and the death of phosphorus-rich organisms is preserved in the sediment by adsorption and complexation with iron oxides and then combined with calcium and fluoride plasma to form collophane; during the early diagenesis process, collophane underwent recrystallization, forming a colloidal, cryptocrystalline, and microcrystalline apatite assemblage.